7 research outputs found
Exact Wave Propagation in a Spacetime with a Cosmic String
We present exact solutions of the massless Klein-Gordon equation in a
spacetime in which an infinite straight cosmic string resides. The first
solution represents a plane wave entering perpendicular to the string
direction. We also present and analyze a solution with a static point-like
source. In the short wavelength limit these solutions approach the results
obtained by using the geometrical optics approximation: magnification occurs if
the observer lies in front of the string within a strip of angular width , where is the string tension. We find that when the distance from
the observer to the string is less than , where is the
wave length, the magnification is significantly reduced compared with the
estimate based on the geometrical optics due to the diffraction effect. For
gravitational waves from neutron star(NS)-NS mergers the several lensing events
per year may be detected by DECIGO/BBO.Comment: 15 pages, 8 figures, reference adde
Cosmological Black Holes
In this paper we propose a model for the formation of the cosmological voids.
We show that cosmological voids can form directly after the collapse of
extremely large wavelength perturbations into low-density black holes or
cosmological black holes (CBH). Consequently the voids are formed by the
comoving expansion of the matter that surrounds the collapsed perturbation. It
follows that the universe evolves, in first approximation, according to the
Einstein-Straus cosmological model. We discuss finally the possibility to
detect the presence of these black holes through their weak and strong lensing
effects and their influence on the cosmic background radiation.Comment: 14 pages, new completely revised version, to appear on GR
On the possible sources of gravitational wave bursts detectable today
We discuss the possibility that galactic gravitational wave sources might
give burst signals at a rate of several events per year, detectable by
state-of-the-art detectors. We are stimulated by the results of the data
collected by the EXPLORER and NAUTILUS bar detectors in the 2001 run, which
suggest an excess of coincidences between the two detectors, when the resonant
bars are orthogonal to the galactic plane. Signals due to the coalescence of
galactic compact binaries fulfill the energy requirements but are problematic
for lack of known candidates with the necessary merging rate. We examine the
limits imposed by galactic dynamics on the mass loss of the Galaxy due to GW
emission, and we use them to put constraints also on the GW radiation from
exotic objects, like binaries made of primordial black holes. We discuss the
possibility that the events are due to GW bursts coming repeatedly from a
single or a few compact sources. We examine different possible realizations of
this idea, such as accreting neutron stars, strange quark stars, and the highly
magnetized neutron stars (``magnetars'') introduced to explain Soft Gamma
Repeaters. Various possibilities are excluded or appear very unlikely, while
others at present cannot be excluded.Comment: 24 pages, 20 figure